A common manufacturing process includes the deposition of mixtures of micron-sized (microscopic, hereinafter) particles. One example is the deposition of a mixture of microscopic particles of TEFLON® and catalyst, such as platinum, on porous carbon substrates used in fuel cells, such as the gas diffusion layer. Another example is diffusion of microscopic carbon and TEFLON® particles on porous carbon substrates so as to provide hydrophobic carbon/carbon substrates (or micro porous layer).
Herein, the devices that enable the deposition of microscopic particles will be referred to as a cloud tower, since typical apparatus resembles a truncated pyramid tower disposed over a vacuum work table, into which the microscopic particles are impelled by inert gas, such as nitrogen.
The cloud tower is fed by a tube or other passageway from material processing apparatus. One example is the formation of a slurry of a desired catalyst and TEFLON®. The slurry is then dried to form pellets, and the pellets are ground into microscopic particles. The pellets are drawn into the hose or other conduit by high pressure inert gas, such as nitrogen, which may be accomplished using an eductor, (sometimes called an ejector).
The work table is either formed of a suitable mesh or has a substantial number of holes therein so as to substantially uniformly apply a vacuum which is attached to the bottom of the work table, to attract and thereby distribute the particles throughout the target area, to draw the inert gas through the pores of the substrate being treated, and for exhaust to atmosphere. Typically, the work table, including the vacuum apparatus, may be raised and lowered in order to place the substrates within the cloud tower for processing; alternatively, the cloud tower itself may be raised due to suitable flexibility in the tube or other conduit.
The cloud towers are custom designed in each case to service a selected size of a sheet of porous carbonaceous material to be processed. Heretofore, the only way to alter the size of the deposition would entail a redesigning of the tower itself in addition to adjusting the points of application of vacuum. While the application of vacuum is easily adjusted, by masking or otherwise, without affecting the process itself (other than the points of application of vacuum), the utilization of a mask within the cloud tower alters the flow distribution of the cloud of mixed microscopic particles, causing wavelets and other distortion in the localized magnitude of distribution. Furthermore, there is local distortion at the mask/substrate interface. These effects easily result in an unwanted variation in the distribution of the particles, and therefore a variation in the degree of activity, for instance, in a substrate having catalyst deposited thereon.
Therefore, means other than the utilization of a mask on a substrate are needed in order to adjust the size or shape of the field of deposition of microscopic particles.